A time domain reflectometer (TDR) is an instrument utilized to detect faults (impedance discontinuities) in constant impedance transmission media, such as two-conductor cable. A TDR detects discontinuities in a length of cable by transmitting a pulse of energy on to the cable; receiving and detecting any reflected energy; and measuring the elapsed time from the transmission of the energy pulse to the receipt of the reflection. Thus, the major components of a TDR include a signal transmitter, a signal receiver, and a signal display device for displaying the transmitted and received signals.
As the transmitted energy pulse from the TDR travels down the cable, all of the pulse energy will be absorbed if the cable is properly terminated and the cable has a constant impedance. If the pulse reaches an impedance discontinuity, part or all of the pulse energy is reflected back to the TDR. As shown in FIG. 1, if the cable fault is an open circuit, the reflected pulse will be in phase (have the same polarity) with the incident pulse. If the cable is short circuited, the reflected pulse will be out of phase (have the opposite polarity) with the incident pulse, as shown in FIG. 2. In either case, a substantial amount of energy is reflected by the discontinuity.
As noted above, a dead short and a complete open both reflect all the pulse energy back from the discontinuity. Other faults that are less severe will reflect varying degrees of energy. A special case, unique to twisted pair cables such as telephone cable, is a bridge tap. A bridge tap is when a second twisted pair has been connected or bridged across an existing trunk line. One example would be where a plurality of party line phones are bridged onto the last subscriber out from the central office. In such a case, three bridge taps would be located along the line, one for each party line phone.
The phase relationship between the incident and reflected pulses is used to determine the type of fault causing the reflection. Reflections from an impedance higher than the characteristic impedance of the cable are in phase. Reflections from a lower impedance are out of phase. The TDR measures the time between the transmitted pulse and reflected pulse to determine the distance to the discontinuity. Use of a digitized waveform, as shown in FIGS. 1 and 2, enables the operator of the TDR to view the signature of the cable in great detail.
Energy pulses travel down different types of cables at different speeds. The speed at which a signal travels is called the velocity of propagation (VOP). VOP is measured as a percentage of the speed of light in free space.
One problem encountered in the testing of twisted pair cable relates to the natural attenuation of a transmitted signal by the resistive, inductive, and capacitive components of the particular cable under test. Because the amplitude of the pulse is reduced by this attenuation, or loss, in the cable, major faults at long distances may appear to be the same as minor faults close to the TDR, as shown in FIGS. 3 and 4. FIGS. 3 and 4 show a cable having the same type but different severity of fault, but because of cable loss, the amplitude of the reflected pulse in FIG. 4 is less than the amplitude of the reflected pulse in FIG. 3. The only difference between FIGS. 3 and 4, is that the location of the fault in FIG. 4 is at a greater distance than that of the fault in FIG. 3.
This natural attenuation present in cables limits the ability of a TDR to test infinitely long cables. The maximum length of cable which may be tested is determined is by: (a) the attenuation per foot at a particular frequency of the cable; (b) the amplitude of the transmitted pulse; and (c) the sensitivity of the TDR's receiver. Because TDRs provide variable pulse widths, the cable loss per foot may be decreased, so as to permit testing of longer lengths of cable, by increasing the pulse width and thus lowering the fundamental frequency of the pulse. The sensitivity of the TDR receiver is determined by the resolution of the TDR's sample and the analog to digital (A/D) sub-system. The pulse amplitude is set at a voltage amplitude which does not overdrive the receiver sub-system. More specifically, the sensitivity of a TDR is defined by the difference in the amplitude of the transmitted pulse, and the amplitude of the smallest reflected pulse which can be detected by the TDR.
Because TDRs have a maximum output voltage, it is not possible to merely increase signal amplitude in order to overcome the loss involved in long lengths of cable. Similarly, the minimum resolvable voltage of the analog to digital converter is fixed by those components which are currently available on the market.
One particular problem found in twisted pair cable, such as is commonly utilized in telephone cable, is that twisted pair cable is particularly lossy at the high frequencies utilized by a TDR. To compound the problem, telephone companies have a propensity for using very long cables.
Cable impedance is determined by the inductive and capacitive electrical components of the particular cable. The loss in the cable is the resistive electrical component of the cable. The inductive and capacitive electrical components of the cable also cause the TDR's high frequency pulses to have an exponential decay. FIG. 5 shows the waveform of a base line, or ideal cable with no exponential decay in broken line as I.sub.B. However, an exponential decay of a cable causes the slope of the incident pulse to lengthen to a very shallow slope, as shown in solid line at I.sub.D.
Thus, exponential decay in a cable greatly enhances the difficulty in detecting a fault at long distances because: (1) the amplitude of the reflected pulse gets smaller, and (2) the reflected pulse gets "lost" in the curve of the exponential decay. As shown in FIG. 6, a reflected pulse R.sub.D has only a very slight change in the slope of the decayed waveform of transmitted pulse I.sub.D. However, the reflected pulse R.sub.B is much more readily apparent in the base line waveform of the "ideal" cable with incident pulse I.sub.B.
A further complicating element lies in the fact that different diameters or gauges of cable have different exponential decay rates, as shown in FIG. 7. Thus, a fault that may be within the range of sensitivity of a TDR on a 19 gauge cable, may not be detectable if the cable is 22 gauge or smaller.
In the case of bridge taps, a special problem for the testing of twisted pair cable arises. A telephone company that wants to convert an old subscriber physical pair to a high frequency T-carrier line may run into bridge taps which can foul the system. Unfortunately, the presence and location of all bridge taps are not always identified in plant records.